1. Field of the Invention
This invention relates to rotating x-ray tubes and more particularly, to high-power, rotating x-ray tubes having improved cathode structures and improved electron-beam deflection systems.
2. Prior Art
X-ray tubes have applications in two fields: medical x-ray diagnostic imaging and industrial x-ray imaging. Medical imaging x-ray tubes are characterized as providing x-rays with high focal spot brightness, or energy per unit area, and a low duty-cycle. Industrial x-ray tubes, which are used, for example, for non-destructive testing (NDT) are characterized as providing x-rays with lower brightness but with high duty-cycles. Medical x-ray tubes often use a rotating target anode enclosed within a vacuum envelope to achieve high peak brightness. The rotating anode is often a disk which is cooled by high-temperature radiation cooling. This radiation cooling must dissipate the heat energy produced, which is typically in the range of 3 kilowatts. Failure to dissipate this heat results in a temperature rise which can irreversibly damage or destroy components of these expensive tubes. The efficiency of radiation cooling dramatically increases at higher temperatures so that efficient radiation cooling requires operation of the anode at high temperatures, which increases the conditions for and the likelihood of tube damage or failure. In contrast, industrial x-ray tubes use a fixed target anode which is being cooled by direct contact with a cooling fluid, permitting high duty-cycles, typically below 3 kilowatts.
Medical x-ray tubes are used in computerized-tomography (CT) imaging systems as a source of high focal spot brightness to form images which are analyzed to precisely differentiate between various tissue structures. However, CT imaging systems, or scanners, have severe operational limitations imposed upon them due to the limited duty-cycles of the rotating anode x-ray tubes used in such CT imaging systems. In operation, because commercial x-ray tubes used in CT systems have very low duty-cycles, these CT systems must be used intermittently in order to allow the x-ray tube to cool. For example, a typical abdominal scan requires 20,000 watts of electron beam power. Yet, the maximum power dissipation, or cooling rate, of a typical rotating-anode x-ray tube is in the range of 1,000 watts, with 3,000 watts power dissipation being available for certain tubes employing an oil-recirculating heat-exchanger. This results in a theoretical maximum duty cycle of 0.05 to 0.15. If, however, a tube is operated at such maximum duty cycle; that is, at its peak anode heat dissipation, tube life is drastically reduced to, e.g., a few hours. So the practical duty cycle is significantly below the quoted theoretical range.
One component which is particularly subject to damage and failure is the bearing supporting the rotating anode within an x-ray tube in a vacuum. Typically, the anode disk is mounted at the end of a rotatable structure supported by the bearing. The bearing surfaces are contained within the vacuum of the tube. Because a lubricant would contaminate the vacuum enclosure, no lubricants are used. Heat dissipation from a tube during high load conditions is provided primarily by radiation of thermal energy from the rotating anode disk to the walls of the envelope containing the vacuum for the tube. The walls of the envelope are composed of glass, metal, and/or ceramic materials and may be surrounded by a dielectric oil bath. For radiation cooling to be effective, the anode disk must be at an elevated temperature. However, if the anode temperature is elevated for an extended period of time, the bearing gets too hot and its lifetime is dramatically reduced. With the advent of CT, the designs of existing rotating x-ray tubes were challenged. Bearings were redesigned to prevent movement of the focal spot, that is, the region on the anode struck by an electron beam, as components of the tube expanded and contracted as the temperature of the tube changed. CT systems were particularly sensitive to movement of the focal spot on a target anode.
Another challenge to tube designers was to increase the average power of a tube to increase its loadability, that is, a tube's ability to handle a greater average power while keeping its temperature within safe limits. Limitations on loadability to a few hundred watts means that a piece of CT equipment must be kept idle to allow the x-ray tube to cool sufficiently to permit a subsequent series of CT scans, or images, to be made. Over the past years the loadability factor has been incrementally improved. It appears that most tube manufacturers have chosen the same solutions to the problems outlined herein. These solutions have involved increasing the diameter, size, weight, and surface emitting of the rotating anode disk, as well as using heat-exchangers for the oil-dielectric surrounding the vacuum envelope of these tubes. Only incremental progress has been made regarding the bearings contained within the vacuum. One manufacturer has introduced a rotating anode tube with liquid bearings.
Currently, the newest and largest-capacity rotating x-ray tubes being commercially produced use heat exchangers and can dissipate up to 3000 watts. Since continuous input powers of 20-30,000 watts are still desired, these x-ray tubes have a duty-cycle of approximately 10% and still must be kept idle for over 90% of the time. Operating these tubes at a power level of 3000 watts reduces the life of their bearings to a few hours. In addition, these tubes with their associated heat-exchangers are quite bulky and very expensive.
Even though x-ray tube designs have been incrementally improved, it still remains a problem that the type of x-ray tubes needed for CT still need to be idled once they have their thermal capacity loaded up by initial operation of the tube from a cold start. In the operation of a CT system, a certain amount of this type of idle time can be masked partially by whatever time is required to perform digital data processing and image reconstruction. As electronic computer processing systems become faster and less expensive, image reconstruction times become shorter and eventually may be the same as the actual x-ray scanning time. However, in certain situations the x-ray tube is still the limiting factor when higher patient throughput is needed, for example, to improve the economic balance sheet of a facility, or to cope with civil emergency situation, or to handle battlefield triage conditions.
Technical x-ray imaging systems do not use rotating anode tubes. For non-destructive-testing (NDT), rotating anode x-ray tubes are rarely used. These systems use so-called stationary anode tubes, which are rugged tubes normally operated at up to a 100% duty cycle and which have substantial service life. This type of tube has a stationary, liquid-cooled anode. However, their peak power is rated at only approximately 2% of the peak power of a rotating anode tube used in medical imaging systems. Since the focal spot remains stationary on the target anode, the power of a stationary anode tube is limited typically to 300 watts for an effective focus size of 1 by 1 millimeter to 50 watts for a 50 micrometer diameter focus size. For applications requiring high spatial resolution, a small focal spot is required and the tube power must be correspondingly reduced. Because their peak power is low, these tubes have severe limitations with respect to their spatial resolution capabilities and with respect to the maximum thickness of an object to be scanned. Industrial x-ray inspection systems are restricted in their performance by the available x-ray tubes. Medical rotating x-ray tubes are inappropriate for this application, because they are not rugged enough, they are expensive, and they are not available for the higher voltages of ten times required for increased penetration of technical objects. Due to the low x-ray output of industrial tubes, the x-ray detection of an industrial imaging system is practically limited to photographic, silver-emulsion-based recording film. Film is an ideal integrator of an x-ray signal, and can thereby compensate for low x-ray flux with long exposure. As a consequence many industrial inspection exposures last for many minutes, or longer. Digital imaging systems requiring a certain minimum flux of x-rays in order to operate above the electronic noise and electronic stability level cannot be used in spite of their other potential advantages already demonstrated in medical imaging.
A number of improved bearings have been proposed for rotating anode x-ray tubes. Also, rotating x-ray tubes are available which use fluid-cooling of the rotating anode, such as, for example, tubes provided by Elliot of England and Rigaku of Japan. These tubes do combine the strong point of the rotating anode tubes (higher peak power capacity) with the strong points of the fixed anode tubes (direct fluid cooling of the anode). However, these tubes are not used in medical imaging systems because the peak performance of these tubes is not equal to that provided by current rotating anode tubes. In addition, these tubes have another disadvantage which is that they are not hermetically sealed. The rotating shaft for the anode goes through the vacuum envelope via a rotary seal which uses a magnetic fluid with a low vapor pressure. The tube needs to be connected to a vacuum pump to maintain and/or establish a high vacuum within the envelope of the tube. This significantly increases the complexity of an imaging system in addition to increasing reliability and cost.
U.S. Pat. No. 4,621,213 for an "Electron Gun" granted to Roy E. Rand on Nov. 4, 1986 describes an electron gun source of electrons for an x-ray tube.
For high-power, rotating x-ray tubes increased accuracy requires improved cathode structures. For a rotating x-ray tube to replace a stationary tube it is required that a compact beam deflection system be provided so that the rotating x-ray tube has the same form factor.